In recent years, wireless LANs have been widely used in offices and homes. In addition, wireless LAN network services that can be used in designated town areas such as train stations, airports, and fast food chain stores have been started. In such wireless LAN network services, wireless stations that are called access points are disposed. The wireless stations as access points are connected to an external backbone network through routers.
When a wireless LAN network service is used, a wireless LAN interface card is attached to an information terminal such as a portable personal computer or a Personal Digital Assistant (PDA). The wireless terminal to which the wireless LAN interface card has been attached is wirelessly connected to a wireless station as an access point. Thus, a portable personal computer and a PDA as a wireless terminal is capable of accessing an external backbone network at high speed.
The service area of one access point of such a wireless LAN network service is at most in the range from several ten meters to several hundred meters. Thus, one access point is incapable of securing a wide service area.
Thus, in the wireless LAN network service, a plurality of access points are geographically continuously disposed so that wireless terminals that move in a wide range are capable of receiving the network service. In addition, the service areas of adjacent access points are overlapped so as to allow a wireless terminal that moves among service areas (called cells) of a plurality of access points to communicate as continuously as possible. In addition, the wireless terminal side monitors communication radio wave states of access points so as to maintain an optimum communication state. If the communication state of the wireless terminal against the current access point deteriorates as it moves, it switches the current access point to another access point that allows the wireless terminal to have a good communication state.
Such an access point switching operation is called a handover. By performing a handover, the wireless terminal is capable of securing a wide service area.
In addition, a wireless LAN network is subject to intercepting and attacks by malicious third parties. Thus, it is necessary to consider communication securities. In a wireless LAN network system using a 4 GHz band, an encrypting mechanism called Wired Equivalent Privacy (WEP) is used. Data to be communicated between an access point and a wireless LAN wireless terminal is encrypted by a designated encryption key. A packet that has not been encrypted with a correct WEP key is rejected so as to assure an access control against an illegal network access.
However, it has been pointed out that the WEP is weak against attacks. (For example, Nikita Borisov states that codes can be relatively easily decoded due to weakness of the WEP. Refer to “Intercepting Mobile Communication—The Insecurity of 802.11”, White Paper, January 2001.) Thus, to prevent a wireless LAN network from being maliciously and illegally accessed, it has been proposed to use IEEE 802.1x“Port-based network access control”. In this system, an authenticating server authenticates a user with his or her user ID and password. An encryption key is dynamically created for each session so as to encrypt data with the created encryption key for a wireless region. In the proposed system, since an encryption key is dynamically created for each session, not fixed, security for data is more improved than the conventional WEP system.
However, in the proposed system, the authenticating server needs to authenticate a user with his or her user ID and password for each session. Thus, when the wireless terminal performs a handover, the load and communication cost on both the wireless terminal side and the wireless station side as access points remarkably would increase.
In other words, as described above, a plurality of access points are geographically continuously disposed so as to secure a wide service area. When a wireless terminal moves, a handover takes place. The wireless terminal switches the current access point to another access point depending on the communication state of the current access point. In the system of which the authenticating server creates an encryption key, whenever an handover takes place and a new session starts, the wireless terminal should be authenticated by the authenticating server through a wireless station as an access point to which the wireless terminal is newly connected. Thus, whenever an handover takes place, it is necessary to re-designate an encryption key for data that the wireless terminal communicates to the wireless station. Thus, the load and communication cost of both the wireless terminal side and the wireless station side as access points would increase.
In addition, in the structure of which the authenticating server encrypts data, when an handover takes place, an application that is operating on the communication wireless terminal is incapable of restoring the communication until the authenticating server has completed the authentication and key designation.
To prevent the load and communication cost from increasing upon occurrence of an handover and an application from interrupting, the Institution Electrical and Electronics Engineers (IEEE) and the Internet Engineering Task Force (IETF) are evaluating a technology of which a wireless station as an access point from which the wireless terminal performs a handover pre-transfers authentication information about the wireless terminal to a wireless station as an access point to which the wireless terminal performs the handover so as to omit operations (authentication and key designation) performed when the wireless terminal performs the handover and is connected to the new wireless station. This technology is generally called context transfer. By performing the context transfer, the wireless terminal is capable of communicating with a new wireless station to which the wireless terminal has performed a handover using the same encryption key used for the wireless station from which the wireless terminal performs a handover.
When the wireless terminal moves in a management area of the same authenticating server, even if the wireless terminal performs a handover, a wireless station from which the wireless terminal performs the handover performs the context transfer to a wireless station to which the wireless terminal performs the handover, the wireless terminal is capable of communicating with the wireless station to which the wireless terminal performs the handover using the same encryption key that has been designated for the wireless station from which the wireless terminal performs the handover. As a result, even if a handover takes place, the load and communication cost can be prevented from increasing and the application from being interrupted.
However, it should be noted that the context transfer is capable of transferring authentication information only when the wireless terminal moves in the management area of the same authenticating server. Thus, when the wireless terminal moves out of the management area of the authenticating server, the wireless station from which the wireless terminal performs the handover is incapable of transferring the authentication information to a wireless station as an access point to which the wireless terminal performs the handover. In other words, only when “authentication information that the wireless station to which the wireless terminal is currently connected transfers a wireless station to which the wireless terminal will be probably connected next” is equal to “authentication information that the wireless terminal is capable of obtaining by causing the authenticating server to authenticate the wireless terminal through the wireless station to which the wireless terminal will be probably connected next”. Otherwise, an authenticating server that manages a wireless station to which the wireless terminal will be connected needs to authenticate it through the wireless station and create an encryption key. Due to this fact, a mechanism that allows a wireless terminal that moves to accomplish and select an authenticating method in accordance with its own moving granularity/moving frequency is required.
The moving granularity represents the physical/logical difference between a subnet from which a mobile wireless terminal moves and a subnet to which it moves. The physical/logical difference is a difference on an access management of whether a plurality of sub nets are provide by the same provider or different providers or a difference on service of whether or not a sub net provides a smooth handoff service that does not interrupt when the wireless terminal moves.